1. Field of the Invention
The present invention relates to a broadband wireless access system, and more particularly, to a method for determining temporary Base Station (BS) Identifiers (IDs) to efficiently set a multi-BS transmission mode and an apparatus for implementing the same.
2. Discussion of the Related Art
MIMO is short for Multiple Input Multiple Output. Beyond conventional schemes using a single Transmit (Tx) antenna and a single Receive (Rx) antenna, MIMO uses a plurality of Tx antennas and a plurality of Rx antennas to thereby increase the transmission and reception efficiency of data. With the use of multiple antennas at a transmitter or a receiver, MIMO seeks to increase capacity and improve performance in a wireless communication system. The term “MIMO” is interchangeable with “multi-antenna”.
The MIMO technology does not depend on a single antenna path to receive an entire message. Rather, it completes the message by combining data fragments received through a plurality of antennas. Because MIMO may increase data rate within a certain area or extend system coverage at a given data rate, it is considered as a promising future-generation mobile communication technology that may find its use in a wide range including mobile terminals, relays, etc. With the growth of data communication, MIMO is attracting attention as a future-generation technology that may overcome a limit on transmission capacity that is almost reached due to the increased data communication.
FIG. 1 illustrates the configuration of a typical MIMO communication system. Referring to FIG. 1, a transmitter has Nt Tx antennas and a receiver has Nr Rx antennas. The simultaneous use of a plurality of antennas at both the transmitter and the receiver increases a theoretical channel transmission capacity, compared to use of a plurality of antennas at only one of the transmitter and the receiver. The channel transmission capacity increases in proportion to in proportion to the number of antennas. Therefore, transmission rate and frequency efficiency are increased. Given a maximum transmission rate Ro that may be achieved in case of a single antenna, the increase of channel capacity may increase the transmission rate, in theory, to the product of Ro and Ri in case of multiple antennas. Ri is a transmission rate increase rate.
For instance, a MIMO communication system with four Tx antennas and four Rx antennas may achieve a four-fold increase in transmission rate theoretically, relative to a single-antenna system. Since the theoretical capacity increase of the MIMO system was proved in the middle 1990's, many techniques have been actively studied to increase data rate in real implementation. Some of the techniques have already been reflected in various wireless communication standards for 3rd Generation (3G) mobile communications, future-generation Wireless Local Area Network (WLAN), etc.
There are two types of MIMO schemes: spatial diversity and spatial multiplexing. Spatial diversity increases transmission reliability using symbols that have passed in multiple channel paths, whereas spatial multiplexing increases transmission rate by transmitting a plurality of data symbols simultaneously through a plurality of Tx antennas. The advantages of these two schemes may be taken by using them in an appropriate combination.
To describe a communication scheme in a MIMO system in detail, the following mathematical model may be used. It is assumed that there are NT Tx antennas and NR Rx antennas as illustrated in FIG. 1. The maximum rank Ri of a channel matrix is given asRi=min(NT,NR)  [Equation 1]
Regarding a transmission signal, up to NT pieces of information can be transmitted through the NT Tx antennas, as expressed as the following vector.s=[s1,s2, . . . ,sNT]T  [Equation 2]
A different transmit power may be applied to each piece of transmission information s1,s2, . . . ,sNT. Let the transmit power levels of the transmission information be denoted by P1,P2, . . . ,PNT, respectively. Then the power-controlled transmission information ŝ may be given as [Equation 3].ŝ=[ŝ1,ŝ2, . . . ,ŝNT]T=[P1s1,P2s2, . . . ,PNTsNT]T  [Equation 3]
ŝ may be expressed as a diagonal matrix P of transmit power.
                              s          ^                =                                            [                                                                                          P                      1                                                                                                                                                                                                                                                                                0                                                                                                                                                                                                                P                      2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                  P                                              N                        T                                                                                                        ]                        ⁡                          [                                                                                          s                      1                                                                                                                                  s                      2                                                                                                            ⋮                                                                                                              s                                              N                        T                                                                                                        ]                                =          Ps                                    [                  Equation          ⁢                                          ⁢          4                ]            
Meanwhile, actual NT transmitted signals x1,x2, . . . ,xNT may be configured by applying a weight matrix W to the power-controlled information vector ŝ. The weight matrix W functions to appropriately distribute the transmission information to the Tx antennas according to transmission channel statuses, etc. These transmitted signals x1,x2, . . . ,xNT are represented as a vector x, which may be determined as
                    x        =                              [                                                  ⁢                                                                                x                    1                                                                                                                    x                    2                                                                                                ⋮                                                                                                  x                    i                                                                                                ⋮                                                                                                  x                                          N                      T                                                                                            ]                    =                                                    [                                                                                                    w                        11                                                                                                            w                        12                                                                                    …                                                                                      w                                                  1                          ⁢                                                      N                            T                                                                                                                                                                                                  w                        21                                                                                                            w                        22                                                                                    …                                                                                      w                                                  2                          ⁢                                                      N                            T                                                                                                                                                                          ⋮                                                                                                                                                                          ⋱                                                                                                                                                                                                                                      w                                                  i                          ⁢                                                                                                          ⁢                          1                                                                                                                                    w                                                  i                          ⁢                                                                                                          ⁢                          2                                                                                                            …                                                                                      w                                                  iN                          T                                                                                                                                                ⋮                                                                                                                                                                          ⋱                                                                                                                                                                                                                                      w                                                                              N                            T                                                    ⁢                          1                                                                                                                                    w                                                                              N                            T                                                    ⁢                          2                                                                                                            …                                                                                      w                                                                              N                            T                                                    ⁢                                                      N                            T                                                                                                                                              ]                            ⁡                              [                                                                                                                              s                          ^                                                1                                                                                                                                                                          s                          ^                                                2                                                                                                                        ⋮                                                                                                                                                    s                          ^                                                j                                                                                                                        ⋮                                                                                                                                                    s                          ^                                                                          N                          T                                                                                                                    ]                                      =                                          W                ⁢                                  s                  ^                                            =              WPs                                                          [                  Equation          ⁢                                          ⁢          5                ]            
where wij denotes a weight for a jth piece of information ŝj transmitted through an ith Tx antenna and the weights are expressed as the matrix W. W is referred to as a weight matrix or a precoding matrix.
The afore-mentioned transmitted signal x may be considered in two cases: spatial diversity and spatial multiplexing.
In spatial multiplexing, different signals are multiplexed prior to transmission. Accordingly, the elements of the information vector s have different values. In contrast, the same signal is transmitted in a plurality of channel paths in spatial diversity. As a result, the elements of the information vector s have the same value.
Spatial multiplexing and spatial diversity may be used in combination. For example, the same signal may be transmitted through some Tx antennas in spatial diversity, while different signals may be transmitted through the other Tx antennas in spatial multiplexing.
Given NR Rx antennas, signals received at the Rx antennas, y1,y2, . . . ,yNR may be represented as the following vector.y=[y1,y2, . . . ,yNR]T  [Equation 6]
When channels are modeled in the MIMO communication system, they may be distinguished according to the indexes of Tx and Rx antennas and the channel between a jth Tx antenna and an ith Rx antenna may be represented as hij. It is to be noted herein that the index of the Rx antenna precedes that of the Tx antenna in hij.
The channels may be represented as vectors and matrices by grouping them. The vector representation of channels may be carried out in the following manner.
FIG. 2 illustrates channels from NT Tx antennas to an ith Rx antenna.
Referring to FIG. 2, the channels from the NT Tx antennas to the ith Rx antenna may be expressed ashiT=└hi1,hi2, . . . ,hiNT┘  [Equation 6]
Also, all channels from NT Tx antennas to NR Rx antennas may be expressed as the following matrix.
                    H        =                              [                                                                                h                    1                    T                                                                                                                    h                    2                    T                                                                                                ⋮                                                                                                  h                    i                    T                                                                                                ⋮                                                                                                  h                                          N                      R                                        T                                                                        ]                    =                      [                                                                                h                    11                                                                                        h                    12                                                                    …                                                                      h                                          1                      ⁢                                              N                        T                                                                                                                                                              h                    21                                                                                        h                    22                                                                    …                                                                      h                                          2                      ⁢                                              N                        T                                                                                                                                          ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                          i                      ⁢                                                                                          ⁢                      1                                                                                                            h                                          i                      ⁢                                                                                          ⁢                      2                                                                                        …                                                                      h                                          iN                      T                                                                                                                    ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                                                  N                        R                                            ⁢                      1                                                                                                            h                                                                  N                        R                                            ⁢                      2                                                                                        …                                                                      h                                                                  N                        R                                            ⁢                                              N                        T                                                                                                                  ]                                              [                  Equation          ⁢                                          ⁢          8                ]            
Actual channels experience the above channel matrix H and then are added with Additive White Gaussian Noise (AWGN). The AWGN n1,n2, . . . ,nNR added to the NR Rx antennas is given as the following vector.n=[n1,n2, . . . ,nNR]T  [Equation 9]
From the above modeled equations, the received signal is given as
                    y        =                              [                                                                                y                    1                                                                                                                    y                    2                                                                                                ⋮                                                                                                  y                    i                                                                                                ⋮                                                                                                  y                                          N                      R                                                                                            ]                    =                                                                      [                                                                                                              h                          11                                                                                                                      h                          12                                                                                            …                                                                                              h                                                      1                            ⁢                                                          N                              T                                                                                                                                                                                                                    h                          21                                                                                                                      h                          22                                                                                            …                                                                                              h                                                      2                            ⁢                                                          N                              T                                                                                                                                                                                          ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            h                                                      i                            ⁢                                                                                                                  ⁢                            1                                                                                                                                                h                                                      i                            ⁢                                                                                                                  ⁢                            2                                                                                                                      …                                                                                              h                                                      iN                            T                                                                                                                                                              ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            h                                                                                    N                              R                                                        ⁢                            1                                                                                                                                                h                                                                                    N                              R                                                        ⁢                            2                                                                                                                      …                                                                                              h                                                                                    N                              R                                                        ⁢                                                          N                              T                                                                                                                                                            ]                                [                                                                  ⁢                                                                                                    x                        1                                                                                                                                                x                        2                                                                                                                        ⋮                                                                                                                          x                        j                                                                                                                        ⋮                                                                                                                          x                                                  N                          T                                                                                                                    ]                            +                              [                                                                                                    n                        1                                                                                                                                                n                        2                                                                                                                        ⋮                                                                                                                          n                        i                                                                                                                        ⋮                                                                                                                          n                                                  N                          R                                                                                                                    ]                                      =                          Hx              +              n                                                          [                  Equation          ⁢                                          ⁢          10                ]            
In the mean time, the numbers of rows and columns in the channel matrix H representing channel statuses are determined according to the numbers of Tx and Rx antennas. The number of rows is identical to that of Rx antennas, NR and the number of columns is identical to that of Tx antennas, NT. Thus, the channel matrix H is of size NR*NT.
In general, the rank of a matrix is defined as the minimum of the numbers of independent rows and columns. Accordingly, the rank of the matrix is not larger than the number of rows or columns. For example, the rank of the matrix H, rank(H) is limited as follows.rank(H)≦min(Nr,NR)  [Equation 11]
If the matrix is eigenvalue-decomposed, its rank may be defined as the number of non-zero eigenvalues. Similarly, in case of Singular Value Decomposition (SVD), the rank may be defined as the number of non-zero singular values. Therefore, the rank of a channel matrix physically means the maximum number of different pieces of information that can be transmitted on given channels.
A different piece of information transmitted in MIMO is referred to as ‘transmission stream’ or shortly ‘stream’. The ‘stream’ may be called ‘layer’. It is thus concluded that the number of transmission streams is not larger than the rank of channels, i.e. the maximum number of different pieces of transmittable information.
The channel matrix H is determined by# of streams≦rank(H)≦min(NT,NR)  [Equation 12]
where “# of streams” denotes the number of streams. One thing to be noted herein is that one stream may be transmitted through one or more antennas.
One or more streams may be mapped to a plurality of antennas in many ways. The stream-to-antenna mapping may be described as follows depending on MIMO schemes. If one stream is transmitted through a plurality of antennas, this may be regarded as spatial diversity. When a plurality of streams are transmitted through a plurality of antennas, this may be equivalent to spatial multiplexing. Needless to say, a hybrid scheme of spatial diversity and spatial multiplexing in combination may be contemplated.
When the above-described MIMO technology applies to downlink transmission, a plurality of Base Stations (BSs) around a Mobile Station (MS) affect downlink transmission to the MS (downlink multi-BS MIMO). Downlink multi-BS MIMO techniques may be considered in two aspects, joint MIMO processing and single-BS precoding with multi-BS coordination, according to transmission schemes.
In the joint MIMO processing mode, a plurality of BSs transmit the same signal to one MS, whereas the single-BS precoding with multi-BS coordination mode is intended to minimize other BS interference with a signal received from a serving BS in a MIMO environment.
To implement the above MIMO techniques in a broadband wireless access system such as an Institute of Electrical and Electronics Engineers (IEEE) 802.16m system, an MS needs to get knowledge of neighbor BSs that transmit signals through multiple antennas. However, a procedure is yet to be specified, for efficiently acquiring information (e.g. BSIDs) about BSs that are involved in or available for participation in a MIMO operation and requesting MIMO transmission based on the acquired information by an MS.